Diaphragm for acoustic devices



Feb. 5, 1935. DUTTON 1,990,066

, DIAPHRAGM FOR ACOUSTIC DEVICES Filed Nov. 15, 1953 2 Sheets-Sheet I II I l I I I I I I 1 I l I I I A TI'OHNE) Feb. 5, 1935. G. F. DUTTON1,990,066

DIAPHRAGM FOR ACOUSTIC DEVICES Filed Nov. 15, 1933 2 Sheets-Sheet 2Patented Feb. 5 1935 UNITED ST S PA EN ,O -I 'E 1,990,066' DIAPHRAGM FORACOUSTIC DEVICES Gilbert Faraday Dutton, South Kensington, London,England, assignor to Electric and Musical Industries Limited, Hayes,England, acompany of Great Britain Application November 15, 1933, No.098,052 4 In Great Britain November 15, 1932 1 20 Claims (01. 181-42 Thepresent invention relates to acoustic de- Kyices and more particularlyto large diaphragms,

that is to say diaphragms ofa size adapted toradiate sound effectivelywithout .the aid of a 5 horn, and to arrangements for supporting suchdiaphragms.

Hitherto large conical diaphragms-have usually been made of paper, andin some cases these have been corrugated. It has usually beenfound thatsuch a paper diaphragm can be made thick fective mass of the diaphragm,and 'hence thefinal response characteristic'curve of the diaphragmwillshow a depression at the frequency at which-a radial mode occurs. v

Variation in radial modes produces bending.

stresses in the diaphragm; hence increasing the thickness of thediaphragm raises the frequency at which the radial mode occurs; and. theintro-. duction of internal friction damps the vibrations. A diaphragmof paper having a density of 0.7 gram per cubic centimetre can be made0.02 inch thick without being unduly heavy. Such a diaphragm is verystiff as regards bending, and this property, together with internalfriction, is sufficient to suppressthe radial modes in the lowerfrequency range, that is below 2,000 cycles per second. If the paper isthinner, for example 0.01 in. thick, radialmodes in the lower frequencyrange tendto become troublesome.

Paper, whether plain or impregnated with hardeningresins, does not havea very high wave transmission velocity. The value of this velocity isgiven by the square root of the ratio. of the Youngs modulus to thedensity. In the best kraft paper the velocity is 1.7 x 10 cm. persecond,and impregnatingthis paper with a hard resin may increase this value to2.5x 10 cm, per second. The high-frequency response of a conicaldiaphragm depends on this value of wave velocity, a low value giving alow cut-off frequency and vice versa. At frequencies exceeding 2,000

cycles per second, wave transmission begins to become apparent in alarge conical diaphragm. It is necessary that this type of transmissionshould occur, since it reduces the effective mass of the diaphragm. Inan ideal diaphragm-wave transmission would take place withoutwave'reflection,

allthe energy being'ra'diated before the wave reached the-boundary ofthe diaphragm.

Hitherto attempts have been made to use aluminium, which has a higherratio of Youngs modulus to density than paper (its wave trans- -mis'sionvelocity, being about 5.1 X 10 cm.- per second), for conical diaphragms,but such 'diaphragms have proved unsatisfactory on account of radialmodes and strong reflections. These defects are due to the high densityof aluminium, compared with paper, which necessitates the use of thinmaterial about 0.0025 in. thick, and to the small internal friction ofsuch a diaphragm. The

radial modes therefore appear at low frequencies and tend to-cause thediaphragm to rattle.

A knownvconical aluminium diaphragm has asmall number'of widelyfspacedconcentric depressions, or half corrugations with uncorrug ated portionsbetween them. This arrangement-is not satisfactory, since reflectionoccurs at the corrugations and the metal between the corrugationsbuckles under the stresses imposed by normal operation and emitsrattlingsounds similar to the noise made by shaking a thin sheet ofmetal. s It-has been proposed to use a conical diaphragm of aluminiumor' aluminium alloy having. continuous corrugations in its surface. The

corrugations were, however, of such a shape and depth that portionsthereof lay substantially in planes normal to the direction of vibrationof the diaphragm. An object of the present invention isto provide adiaphragm which has a good response to the higher as well as to thelower acoustic frequencies. A further object is to provide a metaldiaphragm in which the defects hereinbefore re-' ferred to are obviated.According to the presentinven'tion. a large acoustic diaphragm comprises(that is to sayis lformedsu'bstantially wholly or partly of) a materialhavinga higher ratio of Youngs modulus to density than paper and isprovided with corrugations merging into one another over at least a part.of the surface of thismaterial, the

ratio of the pitch of the corrugations to the depth thereof (measuredfrom trough to crest) being greater than 5 to 1.

According to the invention in a further a s-, pect, a large acousticdiaphragm is formedsubstantially wholly of a material having a higherratio of Youngs modulus to density than paper and provided withcorrugations merging into .radially inner portion formed of a materialhaving a higher ratio of Youngs modulus to density than paper, thisportion being provided with corrugations merging into one another overat least a part of its surface, the ratio of the pitch of thecorrugations to the depth thereof being greater than 5 to 1. Theradially outer portion of this diaphragm is preferably of paper and offrusto-conical form. l

The functions of the corrugations are to re-' move irregularities fromthe surface of the diaphragm, to increase the lowest frequency at whichradial modes of vibration take place and to give the diaphragm thestiffness necessary for safe handling. It is therefore important that,at least in the outer region of the material having a high ratio ofYoungs modulus to density, the corrugations should merge into oneanother and not be spaced widely apart by uncorrugated portions.

It is preferred to make the improved diaphragms of dished form, such asof conical form (which term in the appendant claims is -to be read asincluding a frusto-conical form), and where a dishedform is employed, itis advantageous to make the shape of the corrugations such that allparts thereof lie at a considerable angle to planes normal to thedirection of vibration of the diaphragm.

The region round the driving point, of a conical diaphragm or drivingzone of a frusto-conical diaphragm may be uncorrugated or provided withcorrugations of smaller depth, or longer pitch, or both, than those on aregion farther from the driving point or zone. Where the region roundthe driving point or zone is-uncorrugated the material may be thickerthere than elsewhere. Alteratively in a frusto-conical diaphragm thesmaller diameter end-may have the form of a cylindrical metal spigotflared out of the frustum so as to provide the necessary rigidity.

When vibrated at high frequencies, say from about 1,500 cycles persecond upwards, a cone of, for example, '7 in. diameter acts inthe-manner of a mechanical transmission line, the impedance of whichincreases from the centre outwards. In order to avoid the formation ofpronounced standing waves in the diaphragm, the latter must represent asmooth mechanical transmission line. It is therefore necessary to avoida sudden change of mechanical impedance along the cone radius, such aswould occur if the corrugations were deep. The corrugations shouldconsequently be made as shallow as pos-.

sible, consistent with freedom from rattle. It is also necessary toterminate the mechanical line with a suitable resistive element ofapproximately the same impedance as the cone. In this way the formationof pronounced standing waves due to reflection from the periphery of thediaphragm is prevented.

A preferred arrangement to prevent appreciable refiection from theperiphery is to mount the edge of the diaphragm within a circularaperture in a box or baflle with the aid of an annulus of velvet or thelike, there being interposed between the annulus and the diaphragm alayer of a substance which offers high internal friction to distortiondue to vibrations and which is capable of withstanding considerabledistortion within the elastic limit. A suitable substance is ahighly-plasticized vinyl acetate polymer, a suitable plasticizer beingtricresyl phosphate.

In order to damp vibrations within the diaphragm, it may be formed withone or more joints incorporating a damping substance such as the abovementioned polymer.

The invention will be further described with reference to the examplesshown in the accompan'ying drawings, in which Fig. 1 is a section ofpart of a metal frustoconical diaphragm,

Figs. 2, 3 and 4 are diagrammatic sections of alternative forms ofradial joint in the diaphragm shown in Fig. 1,

Fig. 5 is a section of part of a' moving-coil loud speaker comprising analternative'form of metal frusto-conical diaphragm,

Fig. 6 is a front elevation of a modification of the diaphragins shownin Figs. 1 and 5,

Fig. 7 is a section of part of a. metal and paper frusto-conicaldiaphragm,

Figs. 8 and 9 are diagrammatic sections of aldiaphragm shown' in Fig. 7.

Referring to Fig. l, the diaphragm from a circular sheet of aluminiumabout 8 in. in diameter. After a central disc and a sector have been cutfrom it, the sheet is formedin known manner into a conical frustum byjoining the two radial edges 2 and 3, the apex angle'of the frustumbeing about 105 deg. The joint is made by applying the vinyl acetatepolymer softened by heat and lapping the edges. The frustum is thenplaced in a press and corrugated. While the diaphragm is in the press,the smaller diameter end is drawn into a cylindrical spigot 4 adapted toreceive a driving coil. The corrugations are concentric with the coneand extend from the periphery inwards for about two-thirds of thedistance from the periphery to the spigot, the radially inner portionbeing uncorrugated.

The corrugations are formed by merging arcs of which the radius r is 0.4in. and the depth d is 0.015 in. The pitch p is thus about 0.3 in., andthe ratio of pitch to depth is about 20 to 1. Further, the corrugationsare all of such shape and depth that no parts thereof approachparallelism to planes normal to the direction of vibration of thediaphragm as a whole, that is normal to the axis 5 of the frustum.

1 is formed ternative forms of circumferential joint in the Owing to thereaction following the stretching process of drawing the spigot, themetal of the spigot is put into circumferential compression and themetal in the zone immediately adjoining the spigot is'put intocircumferential tension. This prevents the occurrence of slackness overany small area of the radially inner part of the diaphragm and obviatescrackling noises during operation. 1

The aluminium has the following properties; Breaking stress, 9 to 10-tons per square inch; Youngs modulus, 7.3 x 10 dynes per sq. cm. Forthis aluminium coneof about 7 in. in diameter the preferred thickness is0.0025 in. Radial modes have been found to become prominent indiaphragms of this kind if the thickness-ericeeds 0.004 in. and thediaphragm becomes too fragile if the thickness is below 0.0015 in.

- The value of the square root of the ratio of 1,990,066 Youngs modulusto density for this quality of The driving'coil former 14 is attached tothe aluminium is about 5.1 x10 cm/sec. whereas the corresponding valuefor paper is between about 1.7 x 1-0 'and 2.5. x cm./sec., ashereiribefore mentioned. w

The aluminium used in constructing the above diaphra'gmis -known as.semi-hard. If harder material is used it will be found to have becometoo hard (and therefore liable to fracture in use) after thecorrugations have been formed if these 'are formed in the cold. A softermaterial has not sufiicientstrength to withstand sudden low fre-'quen'cy impulses such as maybe met with in prac 'tice. If thematerial isannealed during the corrugating process it may be possible to use aharder material. In any case endeavour should be made to arrange thatwhen the corrugating process has been finished the'material' of thediaphragmhas.

thehighest. possible degree of hardness consistent with capacity toresist alternating stress without fracture. l v

In order to damp circumferential vibrations,,the

diaphragm shown in-Fig. Imay be modified by the provision of a pluralityof radial 'joints, for example four, spaced at equal intervals, andincorporating a damping substance. These joints may be plain lappedjoints, as previously described I and as shown in Fig. 6.eAlternativeforms of joints are shown in Figs. 2, 3. and 4. In Fig. 2.,the lapped edges 2 and 3 are united by a layer 6 of .fplasticizedvinylacetate polymer reinforced by -thread stitches 7. Fig. 3 shows a foldedjoint,

with the vinyl acetate polymer 6 lying between the folded edges 2 and 3.In Fig. 4 the edges 2 and 3 are substantially butted and joinedby a buttstrap 6' of resistance material;,'which"may be the vinyl acetatepolymer. The diaphragm provided I withsuch joints consists,nevertheless; substanthe spigot .may be shorter,- the necessarystiffness being provided by making the metal-of the rad i-- ally innerzone thicker than elsewhere. v

conical blank may be spun from a sheet of a thick-' ness' equal tothe-maximum thickness of the fin- Thus the isheddiaphragm, the radiallyouter portion being drawn thinner'by the spinning operation. Alter-.natively the conical blank may beformed'from a i 1 xfiat sheet by aseries of-drawi ng operations inter-- 55.

'spersed with annealing operations, by which the;

thickness can be graduated as desired.

"In the modified diaphragm shownin Fig. 5,- which however has notbeenfound quite so satisfactory as that shown-in-Fig, 1, such ajointless -this case thepitch p is about 0.36 in.-,'and the constructionis adopted. The size and material are substantially; the same, but thecorrugations over the outertwo-thirds of the diaphragm-sun:

face are formed'by 'merging arcs of which the radius r is 0.21 in.;.andthe depth' d 0.04 in. In

pitch-to-depth ratio is therefore about .9 to -.1.

The inner one-third of the cone-surface is pro vided withla little morethan one shallow coriu-igation of which the dpth D is 0034131.; formedby mergingarcs of 1.25m. radius R. The pitch is then about 0.95 in.- andthe pitch-to-depth ratio i is 32 to 11' The'thickness of the diaphragmispreferably graded from 0.004'in." at the spigot to 0.002 in. at theperiphery.

radially outer portion 11 of paper.

spigot 4 by cellulose cement, and the leads to the driving coil (one ofwhich is shown and denoted by. 15) are secured to the former l4 and notto the cone, to avoid rattling. The diaphragm.

1" is supported at its centrein known manner by meansof a corrugatedbuckrar'n spider l6 (see Fig. .1) secured byan adhesive such ascellulose cement to the cone; audit is mounted within an apertureB in aboxor battle 9' with the aid of a ring 10 of velvet attached by means ofplasti- "cized' vinyl acetate polymer -6 to the periphery Thevelvet-ring 10 may Fig. 7 shows a further form of diaphragm having aradially inner portion 1' of aluminium and a The maximum diamter isabout '7 in. and the frustum' of paper'll extends from the periphery tothe mid 'point between the periphery and the spigot 4, be-

the same-section as those shown in'Fig; l and extending from the joint12 to about the mid point- The joint 12" between the joint andthe-spigot. shown in Fig.. 8 is lapped, a resistance material, forexample plasticized vinyl acetate polymer, being used as'theadhesive.-'v The joint may, if desired, be reinforced, for example bysewing with thread, similarly to the radial joint shownin Fig. 2.Alternative forms'of resistance joints .between the aluminium andthepaper are shown in Figs. 8 and 9. In Fig. 8 the paperand the aluminiumcones are flanged, the-resistance material: 6 being employed to couplethe flanges to-. I

gether. In Fig. 9 the two portions are lap jointed and secured. by arelatively rigid adhesive 13, such as cellulose cement, the joint beingcoated with. a damping layer of the resistance, material 6.

I claim:

- 1. A large'ac oustic diaphragm comprising a materialhaving a higherratio of Youngs modulus to density than paper, andprovided withcorrug'ations" merging into one another over at least.

a part of the, surface of said material, ther'atio thereofbe-inggreaterthan 5 to 1.

of the pitch. of 'said corrugations to the depth 2. A large acousticdiaphragm formed substantially wholly of a material having a. higherratio of Youngsmodulus to density than paper and provided withcorrugations merging into one another over at least-the radially outerportion of its surface, the ratio of the pitch of said corrugations tothe depth thereof being not less than 3. A large aeoustic'diaphragm theradially inner portion of which is formed of a. material having a higherratioof Youngs modulus to density than paper, saidportion being providedwith corrugations merging into one another over at least'a part of itssurface, the ratio of the pitch .other over at' least apart of the"surface of said .material, the ratio of the pitch of said corrugationstothe depth thereof being greaterth '5 to'l.

.-'5. A large acoustic diaphragm comprising aradially inner portion ofconical form and of the shape of said corrugations being such that.

surface, and a radially outer portion of 'frustoconical form and ofpaper.

7. A large acoustic diaphragm of dished form and provided withcorrugations merging into one another over at least a part of itssurface,

all pafts'thereof lie at a considerable angle to planes normal to thedirection of vibration of the diaphragm.

' 8. A large acoustic diaphragm o frconical'form, comprising ,amaterialhavin'g a higher ratio of Youngs'modulus to density than paper, andprovided with corrugations merging into one another over at least a partof the surface of said material, the shape of said corrugations beingsuch that all parts thereof lie at a considerable angle to planes normalto the direction of vibration of the diaphragm.

9.- A large acoustic diaphragm of conical form, comprising a materialhaving a higher ratio of Youngs modulus to. density than paper, the partof said material in the region round the driving point or zone beinguncorrugated, and the part of said material in a region farther fromsaid point or zone being provided with corrugations merging into oneanother, the ratio of the pitch of said cgrrugations to the depththereof being greater than 5 to l.

10. A large acoustic diaphragmof conical form, comprising a materialhaving a higher ratio of Youngs modulus to density than paper, saidmaterial being provided with corrugations merging into one another overits surface, and said corrugations being of smaller depth in the regionround the driving point or zone than in the region farther from saidpoint or zone.

11. A large acoustic diaphragm of conical form consisting substantiallywholly of a material having a higher ratio of Youngs modulus to densitythan paper, the region round the driving point or zone beinguncorrugated, and the radially outer region being provided withcorrugations merging into one another and having a pitcht -depth ratioof between 15 to 1 and 25 tol.

12. A large acoustic diaphragm of frusto-conical form of which at leastthe radially inner portion is of metal uncorrugated in the region roundthe driving zone and provided with corrugations merging into one anotherover a region farther from said zone, the smaller diameter end of thediaphragm having the form of a cylindrical spigo t flared out of thefrustum in such a manner as to put the spigot into circumferentialcompressionand the metal in the zone immediately adjoining the spigotinto circumferential tension.

13. A large acoustic diaphragm of conical form comprising light metalhaving corrugations merging into one another over at least a part ofsaid the ratio of the pitch to the depth of the corrugations on saidradially outer portion being between 8 to 1 and 30 to 1.

15. A conical acoustic diaphragm of which at least the radially innerportion is of aluminium between 0.0015 and 0.004 in. thick and has anapex angle of between and \deg., the region of said portion more remotefrom the driving point or zone being provided with corrugations merginginto one another and having a pitch-todepth ratio of between 8 to 1 and20 to 1.

16. A large acoustic diaphragm comprising a material having a higherratio of Youngs modulus to density than paper, and a joint incorporatinga substance which offers high internal friction to distortion and whichis capable of withstanding considerable distortion within the elasticlimit, said diaphragm material being provided with corrugations merginginto one another over at least a part of its surface, the ratio of pitchto depth of said corrugations being greater than 5 to 1.

17. A large acoustic diaphragm formed at least in part of a materialhaving a higher ratio of Youngs modulus to density than paper, andcomprising a joint incorporating a highly plasticized vinyl acetatepolymer which serves as a damping substance. I

18. A large acoustic diaphragm of conical form comprising a materialhaving a higher ratio of Youngs modulus to density than paper and ajoint incorporating a highly plasticized vinyl acetate polymer, saiddiaphragm material being provided with corrugations merging into oneanother over at least a part of its surface.

19. A large acoustic diaphragm comprising a material having a higherratio of Youngs modulus to density than paper, and a plurality of radialjoints incorporating a material which offers high internal friction todistortion and which is capable of withstanding considerable distortionwithin the elastic limit.

20. In combination, a large acoustic diaphragm comprising a materialhaving a higher ratio of Youngs modulus to density than paper, and a

